1. Field of the Invention
This invention relates to flatting agents, and more specifically to inorganic hydrogel flatting agents characterized by their high pore volumes, small particle sizes, and narrow particle size distributions.
2. Description of the Prior Art
It is known in the prior art that synthetic or natural particulate materials can be used as flatting agents in various applications such as industrial coatings, synthetic leather, plastics, printing, etc. Ideally, flatting agents should possess the following properties: high pore volume; narrow pore size distribution; appropriate particle size for the particular application; narrow particle size distribution; and maintenance of particle integrity, pore volume and particle size distribution during processing.
Because they possess most of the above properties, precipitated silicas, silica aerogels and xerogels are commonly used as flatting agents. The process for making silica aerogel or xerogel flatting agents is well known to those skilled in the art, and is represented in FIG. 1.
Under this process sodium silicate and sulfuric acid are mixed rapidly and continuously at low temperatures, low pH, and high concentrations to form a hydrosol. The hydrosol sets and undergoes a phase change to a gel-like structure known as a hydrogel. The hydrogel is broken into relatively small sections and washed to remove soluble salts and impurities. It is during this wash cycle that the pore structure of the washed hydrogel is developed. Following the wash cycle, the washed hydrogel is then dried and/or activated by thermal means to form silica gel which can then be ground or milled to a specific particle size and particle size distribution. In some cases, application of a surface treatment to provide lubricity or to improve suspension properties is also performed.
The final gel properties can be controlled by the rate and method of drying. For example, when the hydrogel is dried slowly, the pore structure collapses and results in a xerogel. Xerogels are characterized by a compressed structure, reduced pore volume and relatively high surface area. The pore volumes of flatting types of xerogels are typically around 1.1 ml/g. In contrast, when the liquid in the washed hydrogel is removed by rapid drying, by solvent extraction, azeotropes, or other similar means to reduce the surface tension of the liquid within the hydrogel pores, shrinkage is reduced, the original pore volume is substantially preserved, and an aerogel structure results. Aerogel flatting agents are therefore characterized by their higher pore volumes typically around 1.4 to 1.7 ml/g, and relatively lower surface areas.
There are certain disadvantages in using any previously existing form of silica flatting agent. As previously discussed, xerogels are characterized by their compressed structure and resultant reduced pore volume and thus exhibit reduced flatting efficiency. Aerogels, with their comparatively higher pore volume are among the most efficient flatting agents currently employed in the coatings industry. However, there are substantial capital equipment costs attributable to the water removal, drying and activation processes involved in the manufacturing of aerogel flatting agents. Precipitated silica flatting agents may have flatting efficiencies as high or higher than aerogels, but are inferior otherwise due to their friable nature in terms of maintaining suitable particle size during agitation.
An object of this invention is to provide a flatting agent with improved flatting efficiency.
Another object of this invention is to reduce the number of processing steps in making a flatting agent.
Another object of this invention is to make a flatting agent characterized by its high pore volume, small particle size and narrow particle size distribution.
Another object of this invention is to make a flatting agent, which, when dispersed into a coating vehicle, results in a coating having a fineness of grind greater than 4.75 on the Hegman scale. In accordance with the present invention, there have been provided certain novel flatting agents which comprise inorganic hydrogels having pore volumes greater than 1.0 ml/g, a particle size in the range 1-10 microns and particle size distribution such that when this inorganic hydrogel flatting agent is dispersed in a coating vehicle, the fineness of grind is at least 4.75 on a Hegman scale. The flatting agents of this invention offer an improved efficiency over the prior art materials and involve fewer processing steps than those currently used.
The flatting agents of this invention are prepared by milling an inorganic hydrogel while maintaining a volatiles content of at least 40 weight percent, to produce inorganic hydrogel particles having average particle sizes in the range 1 to 10 microns, pore volumes of at least 1.0 ml/g and a particle size distribution such that when the inorganic hydrogel flatting agent is dispersed in a coating, the fineness of grind is at least 4.75 on a Hegman scale.
Alternatively, the flatting agents of this invention may be prepared by spray-atomizing inorganic hydrosols to form small particles that polymerize to form inorganic hydrogel particles having appropriate pore volume, particle size and particle size distribution as previously defined.
Also provided in accordance with this invention are improved coating compositions comprising a full gloss coating and an inorganic hydrogel flatting agent having a pore volume of at least 1.0 ml/g, an average particle size in the range 1 to 10 microns and a particle size distribution such that when the inorganic hydrogel flatting agent is dispersed in the coating vehicle, the fineness of grind is at least 4.75 on a Hegman scale; and wherein the flatting agent is present in the coating vehicle in from 3 to 15 weight percent loading on a solids basis.
These and other objects will be apparent from the remaining specification and the appended claims.